The main defects which are caused in an optical disk are pin holes in a recording layer and foreign substances absorbed on the surface of the recording layer or mixed in a transparent substrate. To inspect the above defects, a prior art solution has used an optical microscope constructed so that an image is caused by light reflected from the optical disk.
Accordingly, as shown in FIG. 3, pin holes 11 and foreign substances 12 on an optical disk 1 are observed as black shadows (points). Thus the pin holes 11 and the foreign substances 12 have been indistinguishable from one another. Further, in a case where the optical disk 1 is inspected by an optical microscope constructed so that an image is caused by light transmitted through the optical disk 1, even though the pin holes 11 can be observed, it is very difficult to observe the foreign substances 12 because the recording layer, being made of metal, has a low transmittance of light. Still further, coaxial or spiral grooves 2 formed on the optical disk 1 can be observed from the reflected light image, but it is difficult to observe the grooves 2 from the transmitted light image. For this reason, the grooves 2 and the pin holes 11 cannot be observed at the same time with the optical microscope constructed so that the transmitted light image from the optical disk 1 is observed and thus it is impossible to find out the law of causation from investigating the relationship of the grooves 2 with the pin holes 11.
Japanese Published Unexamined Patent Application (PUPA) 61-25041 discloses a bottle inspection apparatus in which the neck and the bottom portions of a bottle are observed by reflected light and transmitted light, respectively. However, in the bottle inspection apparatus, a portion where a reflected light image is caused and a portion where a transmitted light image is caused, are previously separated from each other within a visual field of inspection. Therefore, the bottle inspection apparatus cannot be applied to a usual object to be inspected, as in an inspection of the optical disk, in which a portion (pin hole) through which light transmits and a portion (foreign substance) from which light reflects are mixed.